Nico Dingenouts

Gaussian effective interaction between flexible dendrimers of fourth generation: A theoretical and experimental study

We propose a theory for the effective interaction between soft dendritic molecules that is based on the shape of the monomer density profile of the macromolecules at infinite dilutions. By applying Flory-type arguments and making use of the experimentally measured density profiles, we derive a Gaussian effective interaction whose parameters are determined by the size and monomer number of the dendrimers that are derived from small-angle neutron scattering (SANS) measurements. By applying this theory to concentrated dendrimer solutions we calculate theoretical structure factors and compare them with experimental ones, derived from a detailed analysis of SANS-data. We find very good agreement between theory and experiment below the overlap concentration, where drastic shape deformations of the dendrimers are absent.

Single chain self-assembly: preparation of α,ω-donor–acceptor chains via living radical polymerization and orthogonal conjugation

Alpha,omega-hydrogen donor/acceptor functional polymer strands are prepared via a combination of living radical polymerization and orthogonal conjugation and subsequently self-assembled as single chains to emulate--on a simple level--the self-folding behaviour of natural biomacromolecules.

Analysis of Polymer Latexes by Small-Angle X-Ray Scattering

Small-angle X-ray scattering (SAXS) is a well-established tool of materials science and colloidal physics. It has been applied to a great number of polymeric and colloidal systems [1-5]. This method, however, has not often been applied to latexes. It is interesting to note on the other hand that the analysis of polystyrene latexes with narrow size distributions [6,7] by SAXS played an important role in the early history of this method. A number of early workers in the field of SAXS [8-13] had shown that the minima and maxima seen in the scattering curve of a polystyrene latex (see e.g. p. 54 of ref. [1]) are related to the form factor of a homogenous sphere. Small-angle neutron scattering (SANS) [14,], on the other hand, has been used quite often to analyze the radial structure [15-27] and the surface [28-30] of latex particles. In the case of SANS a high contrast between the particle and the medium or between the different constituents of the particle may be achieved through appropriate substitution of hydrogen atoms by deuterium atoms [14]. Moreover, a variable contrast between the latex particles and the surrounding medium (water) can be adjusted through mixtures of H2O and D2O [14]. This allows the detailed study of the radial structure and surface of the particles through contrast variation which had been earlier established as an investigative tool for polymeric systems [31-37], biological structures (see ref. [38,39] and further citations given there), and in general for colloidal systems [40,41]. In addition, very small scattering angles can be achieved by neutron spectrometer as, for instance, the D11 at the Institut Laue-Langevin [42]. Hence, the magnitude of the scattering vector q (q=(4π/λ)sin(θ/2); λ: wavelength of radiation, θ: scattering angle; [1, 2, 3, 4, 5]) can attain values much smaller than 0.lnm-1 which is necessary for a meaningful analysis of many colloidal svstems.

Analysis of thermosensitive core–shell colloids by small-angle neutron scattering including contrast variation

We present an investigation of thermosensitive core–shell particles by small-angle neutron scattering (SANS). The particles consist of a solid poly(styrene) core and a shell of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. These latex particles are dispersed in water and have a diameter of ca. 150 nm. At ambient temperature the PNIPA-network in the shell is swollen but at higher temperature water is expelled and the shell undergoes a continuous volume transition. The radial extension of the shell is investigated as a function of temperature by use of SANS. The analysis by SANS is performed at different contrasts using appropriate mixtures of H2O and D2O. It demonstrates that the shell has a well-defined compact structure above the volume transition. The swelling of the shell upon cooling can be described in terms of an affine expansion of the network. This is followed by a slight decrease of the volume fraction with increasing distance to the surface of the cores. The analysis by SANS demonstrates that the phase behavior of the network in the shell may be undertaken in terms of average volume fractions. It thus supplements the previous analysis by SAXS in a decisive manner.

Analysis of the correlation of counterions to rod-like macroions by anomalous small-angle X-ray scattering

We present a study of a rod-like polyelectrolyte by anomalous small-angle X-ray scattering (ASAXS). The polyelectrolyte consists of a stiff poly(p-phenylene) backbone with attached positive charged groups that are balanced by bromine counterions. The scattering data are taken far below the absorption edge (13 473.7 eV) and in its immediate neighborhood. The decrease of the measured intensity predicted by theory is directly observed. A new analysis of these ASAXS-data leads to three partial intensities in a numerically self-consistent fashion. In particular, the scattering intensity that is solely due to the cloud of the counterions could be determined and compared to the prediction of the Poisson–Boltzmann cell model. Quantitative agreement is found. ASAXS is thus shown to be a new and highly effective tool for the analysis of polyelectrolytes.

Determination of the structure factor of polymeric systems in solution by small-angle scattering: A SANS-study of a dendrimer of fourth generation

The analysis of concentrated solutions of a dendrimer G4 of fourth generation by small-angle neutron scattering (SANS) is presented. The determination of the structure factor S(q) (q = (4π/λ)sin(0/2); λ: wavelength of radiation; 0: scattering angle) from experimental data is discussed in detail. It is shown that the incoherent contribution I incoh to the measured scattering intensity may profoundly disturb the determination of S(q), in particular at high concentrations of the solute molecules. Contrast variation is used to determine I incoh which is subtracted subsequently from the measured intensities. Moreover, a general method for the check of the data of S(q) for internal consistency is presented. It is based on the expansion of S(q) into powers of q 2 . The structure factor S(q) obtained from the corrected data is shown to be consistent except at the highest concentration of the dendrimer G4 under consideration here. This finding is explained by a change of the form factor of the dendrimers due to overlap of the particles. The method of analysis presented here is general and can be applied to SANS-data of other structures as well.

ZnO nanocontainers: structural study and controlled release

Nanoscale ZnO hollow spheres are prepared via a microemulsion approach. The as-prepared hollow spheres exhibit an outer diameter of 7 nm, a wall thickness of about 2 nm and an inner cavity of 3 nm. The presence of ZnO hollow spheres and their inner cavities is evidenced by electron microscopy (i.e., SEM, STEM, HRTEM). In addition, small-angle X-ray scattering (SAXS) is involved for the first time to reliably prove the shape and structure of the hollow spheres. Moreover, the size and composition of the as-prepared ZnO hollow spheres are characterized by DLS, XRD, FT-IR, UV-Vis and fluorescence spectroscopy. Thiourea (TU) is introduced as a case study to investigate encapsulation/release of molecules in/from nanoscale hollow spheres. Encapsulation of TU is instantaneously performed with the microemulsion-based synthesis and confirmed by FT-IR spectra. Controlled release is possible via addition of acid as well as by ultrasonic treatment (320 W, 35 kHz, 30 min) and validated by characteristic reaction of TU with Bi3+, Cd2+ and Cu2+ to form deeply colored Bi[TU]Cl3, CuS and CdS.

The interface between immiscible polymers in composite latexes : a small-angle x-ray scattering study employing contrast variation

An investigation of the radial structure of composite latex particles by small-angle x-ray scattering (SAXS) is given. Measurements at different contrasts were done by addition of sucrose to the dispersion medium water. The latex particles investigated here consist of a poly(styrene) core and a shell of poly(methylmethacrylate) and were prepared by seeded emulsion polymerization. Since the electron density of both polymers can be easily matched by concentrated sucrose solution, a full analysis of the radial electron density by contrast variation can be given. Depending on the mode of monomer addition during the second polymerization step a very sharp or a diffuse interface between the two incompatible polymers may result.

Residual order in amorphous dry films of polymer latices: indications of an influence of particle interaction

Abstract We report small angle X-ray scattering and atomic force microscopy measurements on macroscopically thick, dry films of polymer latex particles. While the surface of dried droplets has long range order due to layering effects, the overall bulk structure is amorphous. This holds for both low charge polymethylmetacrylate particles and for highly charged polystyrene particles with additional stabilisation with sodiumdodecylsurfate. In the latter case, however, considerable amounts of residual crystal-like order is observed.

Probing the absolute scattering intensity by means of a laboratory-based small-angle X-ray scattering camera using an imaging plate detector

This paper deals with the application of an imaging plate (IP) detector for probing absolute intensities in small-angle X-ray scattering experiments. The IP detector is used with an in-house modified Kratky camera. It is shown that the normalization of scattering data to the transmitted primary beam intensity properly compensates for various unwanted features typically associated with the IP detector, e.g. the fading effect and poorly reproducible scanning times, promoting precise absolute intensity calibration. The absolute intensity calibration was performed by measuring a large number of different silica sols serving as secondary standards. The calibration factor, which converts the measured data into absolute units, was determined by the average over the data obtained for different silica sols, providing a precise calibration of the IP detector. In particular, it is shown that silica sols do not require application of synchrotron radiation or a long exposure time as typically needed if pure liquids or standard proteins are used as secondary standards. To check the applicability of the IP detector in a real synthesis, Stober silica particles were synthesized under various reaction conditions and the scattering intensity was probed and converted into absolute units. The volume fractions and the number densities of silica particles provided by the absolute intensities showed a good agreement with theoretical values predicted for the different reaction conditions. Therefore, the IP detector can be used for accurate probing of absolute intensities.